Crosstalk Between Adipose and Lymphatics in Health and Disease

Abstract

Adipose tissue, once thought to be an inert receptacle for energy storage, is now recognized as a complex tissue with multiple resident cell populations that actively collaborate in response to diverse local and systemic metabolic, thermal, and inflammatory signals. A key participant in adipose tissue homeostasis that has only recently captured broad scientific attention is the lymphatic vasculature. The lymphatic system’s role in lipid trafficking and mediating inflammation makes it a natural partner in regulating adipose tissue, and evidence supporting a bidirectional relationship between lymphatics and adipose tissue has accumulated in recent years. Obesity is now understood to impair lymphatic function, whereas altered lymphatic function results in aberrant adipose tissue deposition, though the molecular mechanisms governing these phenomena have yet to be fully elucidated. We will review our current understanding of the relationship between adipose tissue and the lymphatic system here, focusing on known mechanisms of lymphatic-adipose crosstalk.

Lymphatic Anatomy and Function

Both adipose tissue and lymphatics were, for centuries, primarily distinguished by their respective roles in lipid metabolism. Before the discovery of adipose-derived factors such as leptin in 1994 (1), fat was seen principally as a passive storage site for nutritional lipids. Likewise, ancient Greek anatomists described the lipid-containing milky lymphatic vessels (2), whereas connection to the gut was observed and a role in uptake of dietary nutrients suspected as early as the 17th century (3). The late 17th and 18th centuries saw the discovery and description of nonmesenteric lymphatic vessels (LVs) and their role in fluid absorption (4), whereas in the 19th and 20th centuries, LV participation in immune cell trafficking and cancer metastasis was recognized (5). Only recently, however, have LVs been viewed as more than hollow conduits for dietary lipid uptake and systemic lymph transport; LVs are now known to be active participants in lipid metabolism, immunity, cancer biology, and hypertension, among other physiological and pathological processes (6).

LVs were initially difficult to study given their narrow diameter, nearly translucent walls, and often colorless contents, and some early anatomists attempted to inject them with wax or other substances to better visualize their course (7). We now know that the lymphatic system is a closed, unidirectional network of vessels that drains the tissue interstitium and carries excess fluid, solutes, and cells to the venous vascular system. LVs are present in all vascularized tissues except for bone marrow and the central nervous system (8). In the latter case, interstitial solutes are cleared from the brain parenchyma through the recently described glymphatic system, which runs along the perivenous space to connect the subarachnoid space to the brain parenchyma and drains into to the cervical lymphatic system (9). There is also a proposed relationship between the glymphatic system and meningeal lymphatics that has yet to be fully characterized (10). Because of its pervasive presence in most organs and tissues, lymphatic system homeostasis and its role in disease has gathered increasing interest across multiple disciplines (6, 11).

Anatomically, LVs initiate within the tissue as capillaries tethered by connective filaments to the surrounding tissue (12) which then progressively merge into precollecting vessels followed by secondary collecting vessels (Fig. 1A) and ultimately into the thoracic duct, which empties into the venous system at the junction of the left subclavian and internal jugular veins, or into the right lymphatic trunk in the case of lymph from the right upper arm, thorax, and head (13). As vessels merge, their diameter increases and they are enveloped by a continuous basement membrane and a layer of smooth muscle, which, in concert with contraction of surrounding smooth muscle and local arterial pulsations, propel lymph forward, whereas bileaflet valves prevent backflow (14). The lumen of LVs is lined with a layer of lymphatic endothelial cells (LECs), which in capillaries form discontinuous, discrete “button-like” junctions to allow for entry of fluid, macromolecules, and leukocytes between them (15). When the vessel is filled, it is transiently sealed by overlapping LECs because the intravascular pressure exceeds that of the surrounding interstitium (8). Endothelial junctions become more continuous and “zipper-like” in the collecting vessels to prevent paracellular leakage of solutes and fluid back into the surrounding tissue (15). Collecting vessels may become more permeable in the setting of perinodal adipose tissue inflammation (16).

Figure 1.

Lymphatic system organization and anatomy. (A) Schematic of major lymphatic anatomic and functional characteristics. (B) Confocal image of a perigonadal fat pad whole mount dissected from a mouse in which lymphatic vasculature was labeled by Prox1-CreER-driven tdTomato expression (purple). Scale bar: 300 μm. (C) A lymphatic vessel and valve labeled by NTS-Cre-driven tdTomato (red) in a murine mesenteric fat whole mount. Scale bar: 50 μm. LEC, lymphatic endothelial cell.

There is significant heterogeneity within LEC subtypes that reflects their function within the lymphatic system as well as likely tissue-specific specializations, reviewed in detail previously (17). Both lymphatic capillaries and collecting vessels express certain master lymphatic transcriptional regulators such as Prospero homeobox 1 (PROX1) and signaling receptors like vascular endothelial growth factor 3 (VEGFR3) and neuropilin 2, which bind VEGF-C to maintain LEC identity and growth (18). Lymphatic capillaries express high levels of the hyaluronan receptor LYVE1 (19) and chemokine CCL21 relative to collecting vessels, which allows for the recruitment of CCR7-positive dendritic cells into the lymphatic system (20). Lymphatic valves also have unique transcriptional profiles, including expression of forkhead box protein C2 (FOXC2), a transcription factor required for the formation of collecting lymphatic vessels and valves (21). The recent discovery that Foxo1 deletion in mice results in stimulation of de novo lymphatic valve formation (22) has provided additional insight into the mechanism of LEC specialization. Single-cell RNA-sequencing studies of murine auricular (23) and inguinal, axillary, and brachial (24) lymph nodes have identified multiple LEC subtypes with distinct markers and anatomic niches, and have described LEC type-specific responses to inflammation. Single-cell RNA sequencing has also been performed on human axillary and parotic lymph nodes (25), demonstrating multiple subtypes of LECs.

In addition to interstitial fluid homeostasis, several lymphatic structures are involved in immune surveillance and response, including lymph nodes, the spleen, and Peyer patches in the small intestine. LVs are critical for the trafficking of immune cells out of the tissue to lymph nodes (26). LVs directly participate in adaptive immunity via the ability of LECs to present antigens to T cells (27), for example, and are critical to the coordination of the immune response, as reviewed in detail elsewhere (28). The role of lymphatics in adipose tissue immune cell trafficking is complex and modulated by obesity. For example, adipocyte-specific VEGF-D overexpression promotes adipose tissue lymphangiogenesis, but with different consequences in lean and obese mice. In lean mice, VEGF-D overexpression causes increased local macrophage accumulation and fibrosis (29), likely in part because of the chemoattractant properties of VEGF-D (30), with no impact on systemic glucose homeostasis. This contrasts with VEGF-D overexpression in obesity models, in which amplified lymphangiogenesis results in reduced adipose tissue inflammation and an improvement in systemic glucose metabolism (31). Notably, inflamed macrophages can also produce VEGF-C/D to promote lymphangiogenesis and antigen clearance (32), underscoring the relationship between immune cells and lymphatic function.

Intestinal lymphatics and mesenteric lymph nodes process environmental antigens while adding another critical function: absorption of dietary fat. In the intestine, lymphatic capillaries are called lacteals and are present exclusively in intestinal villi, where they are responsible for absorption of chylomicrons and fat-soluble vitamins (33). The role of the lymphatic system in lipid uptake and transport is reviewed in detail elsewhere (34, 35) and is not the focus of this review, but clearly has relevance to adipocyte biology via regulation of systemic lipid metabolism.

Lymphatic Differentiation and Development

Most LECs have a venous origin (36), except for cardiac LECs, which may derive from a hematopoietic source (37). Notwithstanding their site of origin, LECs are distinct from blood vascular endothelial cells and are regulated by different signals (38). Discovery that expression of Prox1 is required for the development of the lymphatic system in mice revealed a key regulator of lymphatic differentiation as well as provided a transcriptional marker that distinguishes LECs from vascular endothelial cells (39). Overexpression of PROX1 in human adipose derived stem cells promotes a LEC phenotype, capable of forming tubes and junctions typical of LVs (40). Interestingly, PROX1 expression has been reported in human adipocytes, particularly in the omentum (41). In human adipose tissue, LVs are significantly more abundant in visceral depots compared with subcutaneous (42), and accordingly, PROX1 is more highly expressed in human omental adipose compared with subcutaneous (41). Similarly, confocal imaging demonstrates a robust network of lymphatics in mouse perigonadal visceral fat (Fig. 1B).

PROX-1-VEGFR3 signaling is required to maintain a lymphatic progenitor identity during embryonic development (43), and VEGFR3 ligand VEGF-C is an important lymphatic growth factor that can promote LEC hyperplasia (44), lymph node lymphangiogenesis (45), and even ectopic LV formation in bone (46). Factors such as ephrin-B2 (47) and GATA2 (48) potentiate VEGF-C signaling and expression, respectively.

Lipid metabolism plays a unique role in LEC differentiation and LV growth. Fatty acid β-oxidation is required for lymphatic marker expression, and carnitine palmitoyltransferase 1A, a rate-limiting enzyme for fatty acid oxidation, is a target gene of PROX1 and essential for lymphatic development (49). The fatty acid transporter CD36 also plays a key role in LEC function; CD36 knockout mice have shorter lacteals and disrupted tight junctions, and LEC-specific CD36 deletion results in leaky gut lymphatics, obesity, and adipose inflammation (50). Furthermore, lymph nodes are surrounded by adipose tissue, and peri-lymph node adipose tissue has a distinct lipid composition compared to adipose more distant from the node (51), suggesting a possible axis of interaction between lymphatics and adipocyte lipid metabolism.

Following a proliferative phase during development, LECs become quiescent, a state that extends into adulthood under homeostatic conditions (11). Reactivation of LV growth is mostly associated with pathological states such as inflammatory diseases (32). It is unclear whether LECs in different organs have the same proliferative potential or rely on the same signals, though VEGF-C overexpression is sufficient to stimulate lymphatic vessel hyperplasia in multiple tissues, including the skin (52) and skeletal muscle (53), and VEGF-D overexpression in adipocytes stimulates local lymphangiogenesis in brown and white adipose tissue (29).

Lymphatic Function Is Impaired in Obesity

Because of the complexity and plasticity of adipose tissue, metabolic overload and obesity ensnares multiple cell types, including adipocytes (54), macrophages (55), vascular endothelium (56), and mesothelium (57), in a pathological cycle of dysfunction and inflammation. LVs have been implicated as collateral damage in obesity because their function is impaired in both rodent and human obesity. Obesity reduces lymphatic fluid transport and the number and size of lymph nodes and LVs present in the tissue (58), and therefore impairs the drainage of macromolecules from adipose tissue (59) (Fig. 2). This in turn spurs the accumulation of interstitial fluid (lymphedema) (60). Obesity also impacts LV morphology and size (61) and results in decreased contractility (61) and increased permeability, which has been linked to increased inflammation (60) and a subsequent decrease in factors that regulate lymphatic proliferation and function, such as VEGFR3 and PROX-1 (62). Mesenteric lymphatics also exhibit aberrant remodeling in obesity due to pathological VEGF-C signaling, resulting in more branched, disorganized, and tortuous LVs that are associated with lymph leakage into the surrounding visceral fat (63). Mechanical effects of adipocyte hypertrophy on lymphatic flow have been suggested to be a cause of obesity-associated lymphatic dysfunction, though the functional effects of obesity are not limited to LVs within the adipose tissue itself but manifest throughout the body (61).

Figure 2.

The bidirectional and cyclical relationship between lymphatics and adipose tissue. Obesity leads to defects in lymphatic structure and function and an increased risk of lymphedema, while lymphedema leads to abnormal adipose expansion. LV, lymphatic vessel.

Obesity-related lymphatic dysfunction is at least partially reversible. Weight loss (64) and inhibition of T cell-mediated inflammation (65) both improve lymphatic function, and treatment with the cyclooxygenase-2 inhibitor celecoxib prevented mesenteric lymphatic remodeling and reduced local VEGF-C release in a mouse model of obesity (63). Because of these functional effects, obesity increases the risk of developing spontaneous lymphedema (66), which is localized edema that results from impaired interstitial fluid clearance from tissues because of lymphatic incompetence. Obese patients undergoing surgical lymph node dissection are also at increased risk for postoperative lymphedema compared with lean patients (67, 68).

Precise mechanisms of lymphatic dysfunction in obesity remain elusive, though direct adipocyte-LEC interactions, as discussed later, may play a role. There is also evidence that local adipose inflammation may mediate lymphatic pathology in obesity. For example, cytokines IL-4 and IL-13 (69), interferon-γ (70, 71), interferon-α (70), and TGF-β (72) inhibit lymphangiogenesis, and depletion of T cells in vivo improves lymph node vessel formation and dendritic cell recruitment (73). Peri-lymphatic accumulation of inflammatory cells is known to occur in obesity, and treatment with the T-cell inhibitor tacrolimus improves lymphatic pumping function and clearance (65). Interestingly, perilymphatic inflammation is not restricted to intra-adipose lymphatics, but occurs systemically, for example in the ear, trachea, and hindlimb lymphatics in a diet-induced obesity mouse model (65). Obese mice have heightened dermatitis responses to inflammatory skin stimuli as well, which was improved with VEGF-C injection (74). Perilymphatic inflammation also exposes LECs to long-chain free fatty acids in obesity, which increases LEC apoptosis and decreases VEGFR-3 signaling (62).

Lymphatic Dysfunction Promotes Adipose Deposition and Inflammation

The most widely recognized clinical consequence of lymphatic dysfunction is lymphedema. Lymphedema can be due to a primary lymphatic defect, such as in Milroy disease that results from inactivation of the VEGFR-3 gene (75), or secondary to lymphatic vessel disruption, such as following lymph node resection. Lymphedema has a significant clinical footprint, affecting millions worldwide, including at least 20% of women with breast cancer undergoing axillary-lymph node dissections (76), and more than one-third of women undergoing pelvic lymphadenectomy for gynecologic cancer (77). In addition to damaging body image and decreasing quality of life (78), lymphedema and the associated adipose expansion are associated with discomfort and functional impairment, recurrent bacterial and fungal infections, ulcerations, and, in rare cases, cutaneous angiosarcoma (79).

Lymphedema causes significant adipose tissue deposition, predominantly in the subcutaneous compartment (80), though there is subfascial muscle lipid accumulation as well (81). Lymphatic participation in adipogenesis has been suspected for decades (82), supported by early findings that mesenteric lymph supplementation enhances rabbit preadipocyte differentiation in culture (83), later replicated with mouse (84) and human (85) lymphatic fluid and preadipocytes. Adipose-derived stem cells from lymphedematous extremities also demonstrated an enhanced adipogenic potential (86), though this was not replicated in a recent study (85). Malformation of cutaneous lymphatics results in adipose accumulation (87) and idiopathic lymphedema results in dermal lipid accumulation (88). Lymph stasis in a mouse tail lymphedema model results in increased size and number of lipid droplets (89), though the effect of lymphedema on adipocyte size remains unclear. In humans, adipocytes were found to be larger in lymphedematous limbs than in control limbs from the same patient (90), though other studies using unmatched samples have shown conflicting findings regarding adipocyte size (85, 91).

Mouse secondary lymphedema models have indicated that lymphatic fluid accumulation results in induction of the adipogenic program as measured by C/EBPα and PPAR-γ expression (92). In a detailed study of lymphedema-associated adipose tissue in breast cancer survivors, lymphedematous adipose tissue demonstrated upregulation of leptin gene expression as well as altered adipogenic and lipolytic enzymes, though the directionality of those changes was mixed (85). Lymphedema-associated adipose explants and isolated adipocytes had higher isoproterenol-stimulated lipolysis than healthy controls (85). There was also evidence of increased inflammation and fibrosis in lymphedema samples, in concordance with mouse studies (89).

Although several studies have shown that lymph can stimulate adipocyte differentiation, it is important to emphasize that increased adipogenesis is a result—not a cause—of obesity. The first law of thermodynamics stipulates that the change in internal energy of a system, such as an organism, must equal the sum of the energy inputs minus the outputs, and obesity results from a net positive energy balance (ie, the input energy is higher than the output energy) (93) and not from the differentiation of new fat cells. Put more simply, the calories in stored triglycerides must come from somewhere—the body cannot create mass or energy simply by converting 1 cell type (eg, a preadipocyte) to another (eg, an adipocyte). In the setting of overnutrition, adipogenesis is a metabolically advantageous response to a positive energy balance because it allows for fatty acids to be stored safely without causing lipotoxicity (94). Therefore, the effect of lymph on adipose deposition must rely on increased food intake or absorption, reduced energy expenditure, or both. Claims that adipogenesis alone is the cause of obesity in lymphatic dysfunction therefore violate the laws of thermodynamics and must be rejected.

A less well-characterized lymphatic pathology known as lipedema presents with symmetrical and painful fat deposition in a gynoid distribution (95). Because of female predominance and onset with puberty, an estrogen-mediated mechanism has been hypothesized (96), but little definitive evidence has emerged as to the pathophysiology of this condition. Lipedema is characterized by reduced lymph flow (97) and enlarged lymphatic vessels (98) on lymphangiography. Increased serum VEGF-C levels have been noted in lipedema patients compared with body mass index-matched controls, along with higher VEGFR-3 expression in adipose tissue biopsies (99). Descriptions of lymphatic anatomy in lipedema have been inconsistent, ranging from normal (99), to an increase in lymphatic vessel area (100), to the presence of lymphatic microaneurysms (101). Consistently, lipedema-associated fat is associated with exaggerated macrophage infiltration (99, 100, 102). Lipedema-derived adipose stem cells have been demonstrated to exhibit reduced lipid accumulation and adipocyte differentiation in response to in vitro adipogenic stimulation (103, 104), though this finding has recently been called into question because a 3-dimensional spheroid system enabled normal differentiation of these precursors (105). It remains unclear how adipocytes associated with lymphedema and lipedema may be different from the fat expansion and dysfunction seen in standard obesity. Human studies may also be confounded by the underlying body mass index of the patient, as exemplified in 1 analysis, in which differences in lymphatic vessel area were apparent only in obese patients, whereas differences in adipocyte size were only apparent in lean patients (100).

A genetic etiology of lipedema has been suspected based on a self-reported family history of the disease in up to 60% of female patients (106). Molecular mechanisms that underlie lipedema have also been hypothesized based on genetic syndromes in which lipedema is a clinical feature. Interestingly, many of these syndromes have connective tissue components, including Williams syndrome, which results from deletion of the elastin gene (among others) (107), and cutis laxa type III, which results from mutations in the ALDH18A1 gene that encodes the enzyme that catalyzes the reduction of glutamate to Δ ¹-pyrroline-5-carboxylate (108). Additional genetic syndromes associated with lipedema-like adipose deposition have been reviewed in detail elsewhere (109). Genetic testing schema for patients with lipedema have been proposed to assess for related genetic syndromes or overlapping adipose pathologies (109), though this is not commonly pursued in practice. Additionally, an association of lipedema with aortic stiffness has been reported (110), but no mechanistic link has been established.

Transgenic mouse models have also been used to study primary lymphatic diseases, and these models often are notable for adipose tissue dysfunction. Prox1 haploinsufficiency causes adult-onset obesity in mice (84, 111), and LEC-specific CD36 deletion in mice results in disruption of lacteal junctions and visceral obesity, adipose inflammation, and glucose intolerance (50). Although both of these models cause visceral obesity, mice that are haploinsufficient for Vegfr3 have hypoplastic cutaneous lymphatic vessels and demonstrate subcutaneous adipose deposition (112). K14-VEGFR3-Ig mice, which express a soluble VEGFR-3-Ig protein to trap VEGF-C/D under the control of the keratin 14 promoter, lack dermal lymphatic capillaries and exhibit some elements of lymphedema, including subcutaneous adipocyte deposition (113); these mice are protected from obesity and exhibit slightly smaller adipocytes than seen in wild-type mice (114).

An additional clue linking LEC development to adipose tissue function was noted in the study of FOXC2 because inactivating mutations in this gene lead to hereditary lymphedema, whereas a single nucleotide polymorphism in its putative promoter region has been associated with obesity but not type 2 diabetes in both Pima (115) and Scandinavian (116) subjects. Mice overexpressing FOXC2 in adipocytes are protected from visceral obesity (117) and intramuscular fatty acyl coenzyme A accumulation and insulin resistance (118), though these studies did not examine changes in lymphatic function and a direct connection between FOXC2 regulation of lymphatics and adipose tissue has yet to be defined.

There Is an Axis of Communication Between Adipocytes and LECs

Although evidence exists for a relationship between LV and adipose tissue, specific mechanisms of interaction have remained elusive until recently. LECs express receptors for many adipokines, suggesting that direct communication between mature adipocytes and LVs may be mediated via paracrine or endocrine means (Fig. 3). LECs express the leptin receptor, and treatment with high-dose leptin has been reported to negatively impact lymphatic tube formation in culture (119). This contrasts with the literature that describes leptin action on the vascular endothelium, where leptin promotes angiogenesis (120), potentially via increased expression of MMP-2/9 and TIMP-1/2 (121), as well as FGF-2 and VEGF (122). Leptin also increases vascular permeability (122), and its deletion induces neointima formation (123). Much less is known about leptin’s role in regulating the lymphatic vasculature.

Figure 3.

The lymphatic endothelial cell-adipocyte axis. Specific mechanisms of adipocyte-lymphatic endothelial cell (LEC) crosstalk have recently been described. Neurotensin (NTS) is secreted by LECs and can act via NTS receptor 2 (NTSR2) on brown adipocytes to suppress extracellular signal-regulated kinase (ERK) signaling and reduce thermogenesis. In addition to its direct action on adipocytes via β-adrenergic receptors (ADRBs), norepinephrine also stimulates thermogenesis by agonism of the α2-adrenergic receptor (ADRA2) on LECs, which in turn suppresses NTS production, thereby releasing thermogenic components from NTS inhibition. Specific adipocyte-derived factors that affect LEC function include leptin and adiponectin, which have been demonstrated to inhibit and promote lymphangiogenesis, respectively. Additional mediators of the LEC-adipocyte axis are likely to be important in further defining the relationship between adipose tissue and lymphatic dysfunction.

Adiponectin, another secreted product of differentiated adipocytes, stimulates the differentiation of human LECs and promotes tube formation, in addition to mediating LEC nitric oxide production (124). Adiponectin’s role in vascular endothelial function is disputed, and may differ from its role in LECs because adiponectin has been shown both to decrease VEGF-mediated ERK1/2 phosphorylation and endothelial cell proliferation and migration in a human retinal vascular endothelial model (125) as well as promote AMPK phosphorylation and simulate human umbilical vein endothelial cell differentiation and migration (126).

Recently, another axis of LEC-adipocyte interaction was described, linking brown adipose thermogenesis to neurotensin (NTS) produced by LECs (127). In this study, single-cell RNA-sequencing of mouse and human fat revealed a discrete LEC population present in all adipose depots and transcriptionally distinct from vascular endothelial cells. Nts is a specific LEC marker, and NTS-Cre-driven fluorescent labeling is seen in adipose tissue LVs and valves (Fig. 1C). Nts is most highly expressed in LECs of thermogenic brown fat, followed by subcutaneous and perigonadal visceral white adipose tissue, respectively. Nts expression is suppressed by cold and norepinephrine via α2-adrenergic signaling, indicating coordination between the sympathetic nervous system, LECs, and brown adipocytes in regulating thermogenesis. LEC-released NTS was responsible for the antithermogenic actions of NTS, and neurotensin receptor 2 was shown to mediate the dominant effects of NTS on adipocyte metabolism.

In brown adipocytes, NTS activates ERK1/ERK2, which suppresses Ucp1 expression, thereby inhibiting thermogenesis. Treatment of mice with NTRC-824, a small molecule inhibitor specific for neurotensin receptor 2, induced thermogenesis and prevented further weight gain in a diet-induced obesity model, demonstrating impressive systemic consequences for the sympathetic-LEC-adipocyte axis. Although LVs had been previously shown to respond to both α- and β-adrenergic signals (128), and tyrosine hydroxylase-expressing sympathetic nerve fibers are present in lymph nodes (129), this study was the first to indicate direct coordination between these systems and adipocyte function, and strongly suggests that LECs are active participants in adipose metabolism, likely via multiple mechanisms.

Conclusions, Challenges, and Future Directions

The relationship between the lymphatic system and adipose tissue has been increasingly recognized as bidirectional, dynamic, and consequential, though underlying mechanisms have only recently been investigated in detail. Recent findings that LECs express adipokine receptors and that adipocytes respond to LEC-secreted factors are exciting indications that lymphatics are active participants in adipose tissue biology and that modulating this relationship may have implications for human health.

A challenge in studying LECs has been the lack of a specific genetic marker to drive Cre-mediated lineage tracing and gene targeting. There are limitations in cell-type specificity with many LEC models because LECs have diverse developmental origins, and identifying a single genetic driver is difficult (17). For example, as previously discussed, Lyve1 is highly expressed in lymphatic capillaries, but less so in collecting ducts (19), while also being expressed in some macrophage populations (130). Prox1 reliably marks LECs and has a functional role in lymphangiogenesis (131), but is also expressed in developing cardiomyocytes (132), neural stem cells (133), lens cells (134), and hepatocytes (135). Though using a tamoxifen-inducible Prox1-CreER prevents recombination in cell types that only express Prox1 during development, nonspecificity in the adult mouse remains a concern. A promising new Cre model driven by Pdpn (136) is another LEC-specific model that has been developed but is less widely used. Intersectional genetic models using combinations of Cre and Dre drivers may enable more specific targeting strategies, including one designed to target LECs by crossing the Cdh5-Dre with a Prox1-rox-stop-rox-CreER allele (137). This method is comprehensively labeled Prox1 + LECs while avoiding recombination in other tissues, and an LEC-specific gene knockout was highly efficient (137).

Applying single-nucleus RNA sequencing to study lymphedema- and lipedema-related adipose tissue, as has been performed in mouse adipose tissue (138), would enable the network of adipocyte, LEC, immune cell, and other cell type interactions to be further defined, and potentially elucidate the events that trigger and potentiate adipose hypertrophy with lymphatic impairment.

Clinical translation has been limited and few therapies are available to improve lymphatic function or lymphedema. Surgical approaches including vascularized lymph node transplantation (139) and liposuction (140) have proven effective in improving symptoms and quality of life in lymphedema patients, but there is currently no effective medical therapy. Lipedema or other generalized or subtle primary lymphatic defects may contribute to abnormal adipose deposition, but there are currently very few ways to evaluate or treat these conditions in humans, and further translational work still must be pursued. Lymphatic participation in adipogenesis, tissue inflammation, and lipid metabolism makes it a fascinating subject for further investigation into metabolic physiology and disease.

Glossary

Abbreviations

FOXC2

forkhead box protein C2

LEC

lymphatic endothelial cell

LV

lymphatic vessel

NTS

neurotensin

PROX1

Prospero homeobox 1

VEGF

vascular endothelial growth factor

VEGFR

vascular endothelial growth factor receptor

Additional Information

Disclosures: The authors have no conflicts of interest to disclose.

Data Availability

Data sharing is not applicable to this article as no datasets were generated or analyzed.